1. Technical Field
The present invention relates in general to threaded pipe connectors and, in particular, to a pipe connector designed for high fatigue such as offshore well riser connections.
2. Description of the Related Art
In some types of offshore oil and gas production, risers extend from the sea floor to a floating platform for processing and transferring the well fluid to a pipe line. Production risers may be thousands of feet in length and may extend in a long catenary curve from the platform to the subsea well assembly. These risers are subject to tensile loads, bending loads and fatigue due to current and waves. These risers may be in place for years, and a failure can be very expensive to repair.
Generally, there are two types of production riser connections that have been used to produce oil and gas from a subsea wellhead to a surface production tree on a floating offshore platform. Both types utilize a female member having an internal frustoconical circumferential surface provided with a thread, and a male member having an external correspondingly frustoconical circumferential surface and provided with an interfacing thread for engagement with the female member.
In the first type of production riser connection, a single thread is formed in a relatively thick walled, forged member for both the male and female members. These forged members are welded to sections of pipe. The thick wall allows for such features as a highly tapered threads, thread relief grooves, and final cross-sectional areas much greater than the pin. These features can greatly enhance both the static and fatigue strength of the connection. This type of connection also easily accommodates added features such as metal to metal seals and stab guides, both internal and external to the threaded surface. These features greatly enhance pressure integrity and operational characteristics, respectively.
The second type of commonly used production riser connector is referred to as a threaded and coupled connection. In this connection, the female member is threaded in each end of a short coupling sleeve made from either a thick-walled pipe or a forging. The male members typically consist of simply threading the ends of the pipe itself. This type of connection is lighter and less expensive than the one described above. It also eliminates the need for a weld between the connector and the pipe, which eliminates the restrictions on strength and fatigue that is associated with the weld.
Riser connectors of both types described above have a number of disadvantages. For the welded-on connection type, the connection is generally heavy and costly. It also must be welded onto the main pipe body, and therefore becomes limited by the weld itself. The welds are compatible with pipes of limited yield strengths. In addition, the fatigue life of the weld is substantially inferior, in most cases, to that of the connection itself. Therefore this type of connection is limited in both structural and fatigue strength by the pipe weld.
For the threaded and coupled connection, since the entire connection must be formed on the limited cross-section of the pipe, there are significant limits on what can be utilized for the same features that allow weld-on connectors to achieve high levels of performance. Past practices have also utilized thread and seal configurations that were developed for casing applications, where seal integrity from internal pressure and static strength were the main objectives. While connections of this type typically only have slightly limited structural strength, they have significantly reduced fatigue strength. They are also somewhat compromised in the ability to achieve reliable metal-to-metal seals external to the threaded section as well as achieve an effective stab guide in this same location. Thus, an improved threadform for high fatigue threaded and coupled connections would be desirable.